datamodel-code-generator vulnerable to arbitrary local file read via JSON-Schema `$ref` (`file://` and `../` traversal), bypassing `--no-allow-remote-refs`

Summary

datamodel-code-generator resolves JSON-Schema $ref targets that point at the local filesystem without restricting them to the input/base directory and without honoring the remote-reference security control. In the default configuration, an attacker who controls an input schema (a "paste your OpenAPI/JSON-Schema" service, a CI job that generates models from a submitted spec, or any multi-tenant codegen platform) can read any file the process user can read and map the host filesystem. This works via either a file:// absolute URI or a ../-escaped relative reference, and it succeeds even when --no-allow-remote-refs is set.

This is an unauthenticated path-traversal / information-disclosure issue (CWE-22 / CWE-200) plus a bypass of a documented security control.

Details

is_url() classifies file:// as a URL (reference.py:1249):

def is_url(ref: str) -> bool:
    return ref.startswith(("https://", "http://", "file://"))

The remote-ref gate then explicitly exempts file://, so --no-allow-remote-refs never applies to it (parser/jsonschema.py, _get_ref_body):

if is_url(resolved_ref):
    if not resolved_ref.startswith("file://") and self.http_local_ref_path is None:
        if self.allow_remote_refs is False:
            raise Error(...)        # <-- skipped for file://
        ...
    return self._get_ref_body_from_url(resolved_ref)
return self._get_ref_body_from_remote(resolved_ref)

Both local-file branches read the target with no containment check. The file:// branch reads any absolute path:

# _get_ref_body_from_url
if ref.startswith("file://"):
    path = url2pathname(urlparse(ref).path)     # absolute path, anywhere on disk
    return self.remote_object_cache.get_or_put(
        ref, default_factory=lambda _: load_data_from_path(Path(path), self.encoding))

and the plain relative branch lets ../ escape the base directory:

# _get_ref_body_from_remote
full_path = self.base_path / resolved_ref       # no is_relative_to(base_path) check
return self.remote_object_cache.get_or_put(
    str(full_path), default_factory=lambda _: load_data_from_path(full_path, self.encoding))

For contrast, the HTTP-local-ref branch (_get_ref_body_from_local_http_path) does enforce is_relative_to(base_path); these two filesystem branches do not. (This is distinct from the previously reported HTTP $ref/--url SSRF hardened in http.py these code paths never enter http.py.)

The fetched file is read and parsed, yielding two impacts:

1. Arbitrary file read + filesystem oracle (any file). The process opens any readable path, and the three distinguishable outcomes leak filesystem structure: Expected dict, got str/list = the file exists and was read into the process; FileNotFoundError: '<abs path>' = missing; PermissionError: '<abs path>' = exists but unreadable. An attacker uses this to probe arbitrary paths and recover the operator's absolute filesystem layout.

2. Verbatim secret disclosure into the generated code. When the referenced node is JSON-Schema-shaped, values in const/default/enum/description positions are emitted verbatim into the generated Python that is returned to the attacker — i.e. exactly the internal/private schema and config files a codegen host stores (specs with example/default tokens, default credentials, internal hostnames, service-account JSON, k8s secret manifests).

Scope note (to avoid overstating): the raw bytes of unstructured files such as /etc/passwd or a PEM id_rsa are read into the process but are not echoed verbatim into the output, because they parse to a single scalar and are rejected as "not a schema". For those files the impact is the read itself plus the existence/permission/absolute-path oracle; verbatim content disclosure applies to schema-shaped nodes.

PoC

Self-contained reproducer: https://gist.github.com/thegr1ffyn/2a87e81f985883acc30d0118c52da4d3 / (poc.py creates everything in a temp dir, runs the generator, asserts read + leak + gate bypass, and cleans up). Minimal manual reproduction of the arbitrary read (any path works):

printf '{"type":"object","properties":{"x":{"$ref":"file:///etc/passwd"}}}' > probe.json
datamodel-codegen --input probe.json --input-file-type jsonschema --output o.py
# -> "TypeError: Expected dict, got str"  == /etc/passwd was opened and fully parsed.
# Swap the $ref for a missing/unreadable path to see the existence/permission oracle.

Impact

Arbitrary local file read / path traversal (CWE-22) → information disclosure (CWE-200), plus bypass of --no-allow-remote-refs. Any application, CI pipeline, or multi-tenant service that runs datamodel-code-generator on an untrusted schema and exposes (returns, logs, commits, renders) the generated code is affected. The attacker can: open and read any file the process user can read (demonstrated: /etc/passwd, a PEM id_rsa, paths under /root/); map the host filesystem and recover absolute paths; and exfiltrate secret values verbatim from any schema-shaped node. Attacker controls only the input schema; no authentication or special privileges required.

Suggested remediation

1. Do not exempt file:// from the allow_remote_refs gate, treat it as an external scheme.
2. In both _get_ref_body_from_url (the file:// case) and _get_ref_body_from_remote, reject targets that are not resolved.is_relative_to(self.base_path), mirroring the existing check in _get_ref_body_from_local_http_path.

Maintainer status

Confirmed by maintainer review and regression tests. The private fix PR was merged and released in 0.62.0: https://github.com/koxudaxi/datamodel-code-generator-ghsa-8359-h9fx-j6v9/pull/1

Fix summary: apply the remote-ref gate to file:// refs and local JSON Schema refs outside the input base path. In 0.62.0, --no-allow-remote-refs blocks these references; the default compatibility mode emits a FutureWarning and users who intentionally rely on trusted external local refs can pass --allow-remote-refs.

Release status: fixed in 0.62.0 for the documented --no-allow-remote-refs bypass, with a compatibility warning for the default behavior.

Validation: uv run --group test --extra http pytest tests/main/jsonschema/test_main_jsonschema.py passed locally; uv run --group fix ruff check src/datamodel_code_generator/parser/jsonschema.py tests/main/jsonschema/test_main_jsonschema.py passed.

Submitted by: Hamza Haroon (thegr1ffyn)